Positive electrode active material for sodium secondary battery, method of preparing the same, positive electrode for sodium secondary battery, and sodium secondary battery including the same

Abstract

To improve the structural stability of O3-type positive electrode active materials to achieve high capacity and excellent lifespan characteristics.SOLUTION: One embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, including an O3-type sodium composite transition metal oxide containing at least sodium, a transition metal and a doping metal, where the sodium composite transition metal oxide is a secondary particle in which multiple primary particles are aggregated, and the aspect ratio of the primary particles is in the range between 1:1 and 1:2.5.SELECTED DRAWING: Figure 1a

Description

Technical Field

[0001] The present invention relates to a positive electrode active material for a sodium secondary battery, a method for manufacturing the same, a positive electrode for a sodium secondary battery, and a sodium secondary battery including the same.

Background Art

[0002] Lithium ion secondary batteries have been widely used as energy storage devices in various electronic technology fields. In recent years, the demand for lithium ion secondary batteries has increased rapidly, and sodium ion secondary batteries have attracted attention in order to replace expensive metal lithium.

[0003] Since sodium ion secondary batteries have an operating principle of insertion / desorption reaction similar to that of lithium ion secondary batteries, they are one of the next-generation materials with high potential for application to secondary batteries. However, they show low performance in terms of capacity, life characteristics, rate characteristics, etc. compared to lithium ion secondary batteries, and there are difficulties in commercialization. For the commercialization of sodium ion secondary batteries, the development of a positive electrode active material with high performance is an indispensable situation.

[0004] As the positive electrode active material of a sodium ion secondary battery, a layered structure transition metal oxide having a typical simple structure but excellent in electrochemical performance and easy to synthesize is used. Layered structure transition metal oxides are typically classified into O3-type and P2-type according to the crystal structure. The positive electrode active material based on the O3-type structure has a composition such as Na x (TM)O 2 (2 / 3 < x ≦ 1), and the positive electrode active material based on the P2-type structure has a composition of Na x (TM)O 2 (x ≦ 2 / 3).

[0005] In general, O3-type layered oxides have a higher energy density than P2-type layered oxide particles, but have the disadvantage of lower cycle stability due to larger structural changes during charging and discharging. P2-type layered oxides have relatively excellent cycle stability, but have the disadvantages of low sodium content and relatively low energy density, making them difficult to apply commercially.

[0006] However, O3-type oxide particles have Na on the surface of the particles. 2 CO 3 However, sodium by-products in the form of NaOH cause problems such as gas generation due to electrolyte side reactions during battery operation, and reduced capacity and output of the positive active material, reducing the lifespan and stability of the battery. O3-type oxide particles have the problem that when washing with water is applied to remove residual sodium, all the internal sodium escapes and the structure cannot be maintained.

[0007] The present invention aims to realize high capacity and excellent life characteristics by improving the structural stability of the O3-type positive electrode active material. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] China Patent Publication CN 115732674 A [Patent Document 2] Korean Patent Publication KR10-2022-0048915 [Patent Document 3] Korean Patent Registration KR 2228659 B1 Summary of the Invention [Problem to be solved by the invention]

[0009] Another object of the present invention is to provide a method for preparing a positive electrode active material, which can improve battery performance such as structural stability, capacity characteristics, and life characteristics by performing a roasting process and a Cu doping process during the preparation of the positive electrode active material. Another objective is to increase the average particle size of primary particles in O3-type layered oxides, which are secondary particles formed by agglomeration of multiple primary particles, synthesize Cu-doped transition metal oxides, improve air stability and water stability, and alleviate the problems of secondary particle cracking and structural collapse after water washing. [Means for solving the problem]

[0010] An embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, comprising an O3-type sodium composite transition metal oxide including at least sodium, a transition metal, and a doping metal, the sodium composite transition metal oxide being a secondary particle formed by agglomeration of a plurality of primary particles, and the aspect ratio of the primary particles is 1:1 to 1:2.5.

[0011] The ratio (D2 / D1) of the average particle size (D2) of the secondary particles to the average particle size (D1) of the primary particles may be 2.5-10.

[0012] The primary particles may have an average particle size D1 of 0.8 to 2.5 μm, and the secondary particles may have an average particle size D2 of 6 to 12 μm.

[0013] The doping metal may be copper (Cu).

[0014] The sodium transition metal oxide may be represented by the following Chemical Formula 1.

[0015] [ka]

[0016] In the above formula 1, TM is at least one selected from Co, Ni, Mn, and Fe, M is at least one selected from P, Sr, Ba, Ti, Zr, W, Co, Mg, Al, Zn, Ce, Hf, Ta, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and is 0.8 <a<1.0、0.01≦x≦0.1、0≦y≦0.1、0.8≦1-x-y≦0.99である。

[0017] The positive electrode active material may have a (003) peak half width (FWHM(003)) of 0.1599 to 0.3399 at 2θ of 15° to 17.5° in XRD analysis.

[0018] The positive electrode active material may have a residual Na content (TTS, total sodium) of 100 to 3,000 ppm.

[0019] In another embodiment of the present invention, there is provided a method for preparing a positive electrode active material for a sodium secondary battery, the method comprising the steps of: roasting a transition metal hydroxide precursor; dry-mixing the roasted precursor prepared in the roasting process with a doping metal compound; and mixing the metal-doped roasted precursor prepared in the dry-mixing process with a sodium compound in an equivalent amount (Na / M (total metals excluding Na)) of more than 0.8 and less than 1), followed by calcining.

[0020] The roasting step may be carried out at a temperature of 700 to 1,100° C. in an oxidizing atmosphere.

[0021] In the dry mixing step, the doping metal compound may be a copper (Cu) acetate compound, sulfide, nitride, phosphide, oxide, oxyhydroxide, hydroxide or a combination thereof.

[0022] The firing step may be carried out at a temperature of 800 to 1,100°C.

[0023] The method may further include a step of washing the sodium transition metal oxide produced in the calcination step with water.

[0024] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery including the positive electrode active material, and a sodium secondary battery including the positive electrode and a negative electrode. Effect of the Invention

[0025] In the present invention, the cracking of secondary particles can be minimized even after washing with water. In relation to this, the (003) peak full width at half maximum (FWHM(003)), which is the main peak of O3-type, is maintained at the same level as before washing with water, so that the O3-type crystal structure is maintained and residual Na on the particle surface can be removed to a low level. [Brief description of the drawings]

[0026] [Figure 1a] 1 shows the results of SEM-EDS mapping analysis of the surface of positive electrode active material (secondary) particles prepared in Example 1. [Figure 1b] 1 shows the results of cross-sectional SEM-EDS mapping analysis of positive electrode active material (secondary) particles prepared in Example 1 and Comparative Example 1. [Figure 2a] 1 shows comparative SEM analysis results of positive electrode active material particles produced in Example 1 and Comparative Examples 1 to 3 before and after washing with water. [Figure 2b] 1 shows comparative SEM analysis results of positive electrode active material particles produced in Example 1 and Comparative Examples 1 to 3 before and after washing with water. [Figure 3a] 1 shows comparative XRD analysis results of positive electrode active material particles produced in Example 1 and Comparative Examples 1 to 3 before and after washing with water. [Figure 3b] 1 shows comparative XRD analysis results of positive electrode active material particles produced in Example 1 and Comparative Examples 1 to 3 before and after washing with water. [Figure 4] 1 is a graph showing the residual Na content (TTS, total sodium) on the surface of positive electrode active materials after washing with water in Example 1 and Comparative Examples 1 to 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The advantages and features of the present invention, and the methods for achieving them, will become apparent from the following detailed description of the embodiments in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms, and the embodiments are provided to make the disclosure of the present invention complete and to allow those skilled in the art to fully understand the scope of the invention, and the present invention is defined only by the scope of the claims.

[0028] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used as commonly understood by those having ordinary skill in the art to which the present invention belongs. Throughout the specification, when a part is described as "comprising" a certain component, this does not mean that it excludes other components, but that it may further include other components, unless otherwise specified to the contrary. In addition, the singular form includes the plural form unless otherwise specified in the context.

[0029] An embodiment of the present invention provides a positive electrode active material for a sodium secondary battery, comprising an O3-type sodium composite transition metal oxide containing at least sodium, a transition metal, and a doping metal, the sodium composite transition metal oxide being a secondary particle formed by agglomeration of a plurality of primary particles, the primary particles having an aspect ratio of 1:1 to 1:2.5.

[0030] In general, it is known that O3 oxide anode materials have poor air stability. Specifically, when exposed to air or in contact with water, the oxidation reaction of water and H + / Na + An exchange occurs, NaOH, Na 2 CO 3 Residual Na such as CO is generated on the surface. 3 is embedded in the transition metal layer. 4The formation of tetrahedrons slows down the Na diffusion and deteriorates the electrochemical properties. Therefore, the weakened bonding between TM-O can be effectively regulated by the introduction of doping metals to effectively regulate the charge transfer between Na and O, resulting in a stronger Na 2 It can build up O bond energy. It has structural stability and excellent air stability.

[0031] In addition, in the present invention, in order to realize the aspect ratio of the primary particles, a roasted precursor is prepared by performing roasting at a high temperature, and then a doping metal is dry-mixed to be uniformly dispersed on the surface of the roasted precursor particles.

[0032] In the present invention, the roasted precursor oxidized and roasted at high temperature is used, so that the size of the primary particles increases, the specific surface area decreases, and the density of the primary particles aggregated to the secondary particles increases. In this case, (1) the bonding force between the primary particles increases, so that the particle cracking can be improved even after washing with water, and the cohesive force between the primary particles is improved, so that the desorption of Na from the crystal structure due to damage caused by washing with water can be prevented. In addition, (2) due to the characteristics of the doping metal used in a small amount, there is an effect that the doping dispersion is relatively increased on the surface of the primary particles with a reduced specific surface area and the surface of the secondary particles. Therefore, compared with the purpose or effect of improving the surface characteristics mainly through conventional particle surface doping, in the present invention, the doping metal can be uniformly positioned inside the secondary particles and at the crystal grain boundaries of the primary particles, so that the structural stability of the O3-type sodium composite transition metal oxide can be further improved.

[0033] In the present invention, specifically, the aspect ratio of the primary particles may be 1:1 to 1:2.5, for example, 1:1 to 1:2.4, 1:1 to 1:2.3, 1:1 to 1:2.2, 1:1 to 1:2.1, 1:1 to 1:2, 1:1 to 1:1.9, 1:1 to 1:1.8, 1:1 to 1:1.7, 1:1 to 1:1.6, and preferably 1:1 to 1:1.5. In the present invention, the oxidation roasting process of the cathode active material precursor may be performed at a high temperature so that the aspect ratio range of the primary particles can be realized. In addition, by performing the roasting and doping stepwise under specific conditions, the size of the primary particles constituting the secondary particles in the cathode active material can be increased and the aspect ratio can be reduced, thereby providing a cathode active material having improved energy density, high voltage stability, life characteristics, and high rate characteristics.

[0034] The ratio (D2 / D1) of the average particle size (D2) of the secondary particles to the average particle size (D1) of the primary particles may be 2.5 to 10, for example, 2.5 to 8, 2.5 to 6, or 2.5 to 5. When the size ratio (D2 / D1) of the primary particles is more than 10, the size of the primary particles is too small, so that the primary particles may not be formed in terms of shape, and therefore the O3 structure may not be formed and a large amount of by-products (impurity) may be generated. These results are due to the roasting reaction not proceeding smoothly or the O3 structure not being crystallized, for example, due to roasting at a low temperature, roasting for a short time, and / or non-uniform roasting. Meanwhile, the size of the primary particles may be the length of the major axis.

[0035] The average particle size D1 of the primary particles may be 0.8 to 2.5 μm, for example, 0.8 to 2.3 μm, 0.8 to 2 μm, 0.8 to 1.7 μm, or 1 to 1.5 μm. The average particle size D2 of the secondary particles may be 6 to 12 μm, for example, 6 to 10 μm, or 6 to 8 μm. The primary particles and the secondary particles contained in the positive electrode active material can improve the particle density in the positive electrode active material by satisfying at least the above conditions. This can improve the electrochemical properties of the positive electrode active material.

[0036] Meanwhile, the term "aspect ratio" used in the present application refers to the ratio (Length / Width ratio) of the long axis (Length) to the short axis (Width) of the primary particle, and when the long axis indicates the direction of a relatively long region of the primary particle, the short axis indicates the length of a relatively short region located on the same plane as the long axis. In this case, the primary particle may have a plate shape, and the length in the thickness direction of the primary particle is significantly smaller than the length in the plane direction (long axis and short axis) of the primary particle. Meanwhile, the short axis may be a direction perpendicular to the long axis, and the "aspect ratio" of the primary particle can be calculated as the ratio of the long axis to the short axis of the primary particle measured from the surface of the primary particle.

[0037] On the other hand, in the present invention, 50% or more, for example 60% or 70% or more of the total number of primary particles constituting the secondary particle may have the aspect ratio, primary particle size, and ratio of the size of the secondary particle to the size of the primary particle (D2 / D1) within the above-mentioned ranges, or at least 10 or at least 20 of the primary particles constituting the secondary particle may have the aspect ratio, primary particle size, and ratio of the size of the secondary particle to the size of the primary particle (D2 / D1) within the above-mentioned ranges.

[0038] The doping metal may be copper (Cu). The Cu doping can replace part of Fe and Ni among the transition metals, thereby reducing the amount of Fe and Ni that migrates. In addition, the Cu doping can improve the average valence state of Mn through charge compensation, and can reduce the amount of Mn. 3+The Jahn-Teller effect can be reduced. Also, when compared with the case where Co is uniformly doped, Cu tends to be doped non-uniformly due to its own characteristics. However, in the present invention, there is an effect that the doping dispersion degree increases relatively at the surfaces of primary particles with a reduced specific surface area and secondary particles. Therefore, the non-uniformity of Cu doping can be significantly improved. As a result, although the O3-type oxide particles have a problem that when water washing is applied to remove residual Na, all the internal Na leaks out and the structure cannot be maintained, air stability and water stability can be improved, and the problems of cracking and structural collapse of secondary particles after water washing can be improved.

[0039] Specifically, the sodium composite transition metal oxide may be represented by the following Chemical Formula 1.

[0040]

Chemical Formula

[0041] In Chemical Formula 1, TM is at least one selected from Co, Ni, Mn, and Fe, M is at least one selected from P, Sr, Ba, Ti, Zr, W, Co, Mg, Al, Zn, Ce, Hf, Ta, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd, and 0.8 ≦ a ≦ 1, 0.01 ≦ x ≦ 0.1, 0 ≦ y ≦ 0.1, 0.8 ≦ 1 - x - y ≦ 0.99.

[0042] When the Na equivalent of the O3-type layered oxide becomes less than 0.80 (a < 0.80), the electrochemical properties may deteriorate due to the arrangement of lattice units because the oxide has a P3-type structure. Conversely, when the Na equivalent exceeds 1.0 (0.1 < a), there are disadvantages that the air and moisture stability decreases and it is sensitive to synthesis conditions such as temperature and atmosphere.

[0043] The positive electrode active material of the present invention may have a (003) peak half width (FWHM(003)) of 0.1599 to 0.3399 at 2θ of 15° to 17.5° in XRD analysis. The result of the XRD analysis may be a result of analysis after washing with water to remove residual Na on the surface of the produced positive electrode active material. As a result, the positive electrode active material can maintain the O3 type crystal structure well without particle cracking or structural collapse even after removing residual Na on the surface by washing with water.

[0044] In the positive electrode active material of the present invention, the residual Na content (TTS, total sodium) may be reduced to 100 to 3,000 ppm, specifically, 500 to 3,000 ppm, which can suppress gas generation caused by residual Na and improve battery life characteristics.

[0045] On the other hand, the content of residual sodium (TTS, Total Sodium) is the residual amount of sodium-containing compounds (e.g., NaOH or Na 2 CO 3 ), a value obtained by separately calculating the total amount of Na only (TTS, Total Sodium).

[0046] Another embodiment of the present invention provides a method for preparing a positive electrode active material for a sodium secondary battery.

[0047] The preparation method includes a step of roasting a transition metal hydroxide precursor, a step of dry-mixing the roasted precursor prepared in the roasting step with a doping metal compound, and a step of mixing the metal-doped roasted precursor prepared in the dry-mixing step with a sodium compound in an equivalent amount of Na / M (total metals excluding Na) greater than 0.8 and less than 1, followed by calcination.

[0048] The roasting process is a process of heat-treating a transition metal hydroxide precursor at a high temperature. When a hydroxide precursor is doped with a transition metal by a dry method without roasting, a particle crack phenomenon may occur in the oxide secondary particles or primary particles after a heat treatment for sodium insertion in a post-process. This phenomenon is analyzed to be caused by organic elements such as acetate, sulfide, nitride, and phosphide, or oxygen elements such as oxide, oxyhydroxide, and hydroxide, among the anion groups contained in the doping compound. In addition, when a transition metal is doped by a wet method, the selection of the doping compound is limited, and there is a problem of increased costs due to the complicated process.

[0049] The roasting process may be performed in an oxidizing atmosphere at a temperature of 750 to 1050°C, preferably 800 to 1050°C, 800 to 950°C, 850 to 1000°C, or 850 to 950°C. If the roasting process is performed at a temperature higher than the above range, the size of the synthesized roasted precursor primary particles may become too large, which may limit Na ion diffusion on the particle surface. On the other hand, if the roasting process is performed at a temperature lower than the above range, the primary particles may not grow sufficiently, which may make it difficult to realize the desired aspect ratio of the primary particles. In this case, the roasting time is not particularly limited, but may be preferably performed for 6 to 15 hours, 8 to 15 hours, or 8 to 13 hours.

[0050] When the roasting process is performed, the specific surface area and porosity of the roasted precursor are reduced, and the particle size of a plurality of primary particles in a secondary particle is increased, resulting in a phenomenon of high cohesion, which increases the tap density of the roasted precursor and prevents the problem of particle cracking that occurs when dry doping is performed without roasting.

[0051] The transition metal hydroxide precursor may be represented by the following Chemical formula 3, and the roasted precursor may be represented by the following Chemical formula 4.

[0052] [ka] (TM is at least one selected from Co, Ni, Mn, and Fe)

[0053] [ka]

[0054] (TM is at least one selected from Co, Ni, Mn, and Fe)

[0055] The dry mixing process is a process of dry mixing the roasted precursor produced in the roasting process with a doping metal compound. When dry doping is performed after the roasting process, the primary particles have a dense shape through roasting, and the binding force between the primary particles increases, improving the metal doping effect (cohesion between primary particles). In addition, when Cu doping is performed without roasting, some of the structure after washing with water is maintained, but cracks are likely to occur between the primary particles.

[0056] By applying the dry method, Cu can be uniformly doped on the surface and inside of the roasted precursor particles. On the other hand, when doping a transition metal by a wet method, the selection of doping compounds is limited and the process becomes complicated, which increases costs, which is not preferable.

[0057] The copper (Cu) compound may be a copper (Cu) acetate compound, a sulfide, a nitride, a phosphide, an oxide, an oxyhydroxide, a hydroxide, or a combination thereof.

[0058] The calcination process is a step of mixing the copper (Cu)-doped roasted precursor and a sodium compound in an equivalent amount of Na / M (total metals excluding Na) greater than 0.8 and less than 1 to produce an O3-type sodium transition metal oxide, and then calcining the mixture.

[0059] The copper (Cu)-doped roasted precursor and the sodium compound may be mixed at Na / M (total metals excluding Na) = 0.8 to less than 1 equivalent or 0.8 to less than 0.95 equivalent. When the amount of sodium compound mixed is within the above range, the prepared positive electrode active material may have an O3-type layered crystal structure, and thus has a higher energy density, high air and moisture safety, and is less sensitive to synthesis conditions (temperature, atmosphere, etc.). In addition, within the above sodium content range, the battery discharge capacity can be improved and the amount of unreacted residual Na can be minimized.

[0060] The calcination may be performed at a temperature of 700°C to 1,100°C. When the calcination temperature is within the above range, the reaction between the raw materials occurs sufficiently, and the particles can grow uniformly. The calcination may be performed at a temperature of more preferably 750 to 1,050°C, 850 to 1,050°C, or 900 to 1,000°C. The calcination may be performed for 5 hours to 40 hours. When the calcination time is within the above range, a highly crystalline positive electrode active material is obtained, the particle size is appropriate, and production efficiency can be improved. The calcination may be performed for more preferably 5 to 20 hours, 5 to 18 hours, 8 to 15 hours, or 10 to 14 hours.

[0061] The sodium compound is Na 2 CO 3 , NaOH, NaNO 3 , C.H. 3 COONa and Na 2 (COO) 2 At least one selected from the group consisting of, preferably, Na 2 CO 3 , NaOH or a combination thereof.

[0062] Another embodiment of the present invention provides a positive electrode for a sodium secondary battery and a sodium secondary battery, each including the positive electrode active material.

[0063] The positive electrode includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, and the positive electrode active material according to one aspect of the present invention is present in the positive electrode active material layer.

[0064] The positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity, and may be, for example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. The positive electrode current collector may usually have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesive force of the positive electrode active material. Such a positive electrode current collector may be provided in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0065] The positive electrode active material layer may be a layer containing a conductive material and a binder in addition to the above-mentioned positive electrode active material.

[0066] Here, the conductive material is used to give conductivity to the electrode, and can be used without any particular limitation as long as it does not cause a chemical change in the positive electrode active material and has conductivity. Non-limiting examples of the conductive material include graphite such as natural graphite and artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon-based materials such as carbon fibers, metal powders or metal fibers such as copper, nickel, aluminum, and silver, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and conductive polymers such as polyphenylene derivatives. The conductive material may be generally included in an amount of 1% by weight to 30% by weight based on the total weight of the positive electrode active material layer.

[0067] The binder is a material that plays a role in improving the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the current collector. Non-limiting examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene-polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. The binder may be typically included in an amount of 1 wt% to 30 wt% based on the total weight of the positive electrode active material layer.

[0068] The positive electrode according to an embodiment of the present invention may be manufactured by a method for manufacturing a positive electrode for a sodium secondary battery, except that the positive electrode active material is used. For example, a positive electrode active material layer forming slurry including a positive electrode active material and, optionally, a binder and a conductive material may be applied onto a positive electrode current collector, followed by drying and rolling to manufacture the positive electrode. According to another example, the positive electrode active material layer forming slurry may be cast onto a separate support, and the positive electrode active material layer may be peeled off from the support, and the resulting film may be laminated onto a positive electrode current collector to manufacture the positive electrode.

[0069] According to yet another aspect of the present invention, there is provided an electrochemical device including the above-mentioned anode, wherein the electrochemical device may be specifically a battery, a capacitor, or the like, more specifically a sodium secondary battery.

[0070] The sodium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte (electrolytic solution). The sodium secondary battery may also include a battery container (case) that houses the electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0071] Depending on the shape of the battery container (case), sodium secondary batteries can be classified into can-type sodium secondary batteries, in which the electrode assembly is housed in a metal can, and pouch-type sodium secondary batteries, in which the electrode assembly is housed in a pouch made of a sheet such as an aluminum laminate.

[0072] In particular, in the case of a pouch-type sodium secondary battery using a positive electrode including the positive electrode active material according to various embodiments of the present invention, there is an advantage that the possibility of a side reaction between the positive electrode active material and the electrolyte is low, thereby improving stability during storage and / or operation and reducing gas generation.

[0073] Hereinafter, the present invention will be described in detail with reference to examples. However, these examples are provided for the purpose of explaining the present invention in more detail, and the scope of the present invention is not limited to the following examples.

[0074] Working Example Example 1 Ni 0.33 Fe 0.33 Mn 0.33 (OH) 2 The NFM11 precursor was placed in an alumina crucible, and then oxidized and roasted at 950°C for 6 hours in an air atmosphere. The roasted precursor (Ni-Fe-Mn)O was then cooled to room temperature. 4 was manufactured.

[0075] The prepared roasted precursor and Cu(OH) 2 The materials were mixed at Cu / M (M = Ni + Fe + Mn + Cu) 2 at mol% using a hand mixer and dry doped.

[0076] The prepared Cu 2 at mol% doped roasted precursor and Na 2 CO 3 The mixture was mixed in an amount of Na / (Ni+Fe+Mn+Cu) = 0.85 equivalents to obtain a mixture. The mixture was placed in an alumina crucible and O 2 After sintering at 950℃ for 6 hours under atmospheric conditions, it was cooled to room temperature and the O3-type Na 0.85Ni 0.33 Fe 0.31 Mn 0.33 Cu 0.02 O 2 A positive electrode active material was produced.

[0077] The prepared positive electrode active material was put into a reactor containing distilled water, washed with water at a temperature of 5 to 50° C. and a stirring speed of 350 rpm for 1 hour, and dried under vacuum at a temperature of 120° C. for 12 hours.

[0078] The positive electrode active material (85 wt%), carbon black (10 wt%), and PVDF binder (5 wt%) were dispersed in N-methyl-2-pyrrolidone (NMP) to prepare 30 g of positive electrode slurry. The positive electrode slurry was uniformly applied to an aluminum thin film having a thickness of 15 μm and dried in a vacuum at 135° C. to prepare a positive electrode for a sodium secondary battery.

[0079] A sodium metal plate was used as a counter electrode for the anode, and porous glass fiber (thickness: 200 μm) was used as a separator. NaPF was dissolved in a solvent of propylene carbonate and fluoroethylene carbonate mixed in a volume ratio of 98:2. 6 A sodium secondary battery (coin cell) was manufactured using an electrolyte containing 1.0 M of sodium hydroxide.

[0080] Comparative Example 1 A positive electrode active material and a sodium secondary battery were manufactured in the same manner as in Example 1, except that the oxidation roasting process was not performed.

[0081] Comparative Example 2 Those that have not undergone the oxidative roasting process,

[0082] NFM11 precursor and Cu(OH) 2 Cu 2 at.mol% and Na 2 CO 3 Mix the above at the same time in an amount of Na / (Ni+Fe+Mn+Cu) = 0.85 equivalents, then place in an alumina crucible and add O 2A positive electrode active material and a sodium secondary battery were manufactured in the same manner as in Example 1, except that the mixture was sintered at 950° C. for 6 hours under atmospheric pressure.

[0083] Comparative Example 3 A positive electrode active material and a sodium secondary battery were manufactured in the same manner as in Example 1, except that the oxidation roasting process and the Cu dry doping process were not performed.

[0084] Comparative Example 4 Cu(OH) 2 Instead of Co(OH) 2 A positive electrode active material and a sodium secondary battery were manufactured in the same manner as in Example 1, except that the above-mentioned was used.

[0085] Experimental Example Experimental Example 1: SEM-EDS mapping analysis of positive electrode active material particles The surface of the positive electrode active material particles prepared in Example 1 was subjected to SEM-EDS mapping analysis, and the result is shown in FIG. 1a.

[0086] Cross-sectional SEM-EDS mapping analysis was performed on the positive electrode active material particles prepared in Example 1 and Comparative Example 1, and the results are shown in FIG. 1b.

[0087] In FIG. 1a, it can be seen that Cu partially aggregates on the surface of the secondary particles of the positive electrode active material of Example 1, but that Cu is doped relatively uniformly over the entire surface of the secondary particles.

[0088] In FIG. 1b, it can be seen that Cu is unevenly aggregated inside the secondary particles and on the surface of the primary particles. However, in the case of Example 1, when compared with Comparative Example 1, the size of the primary particles increases (see FIG. 2b), and therefore Cu is dispersed on the surface of the primary particles with increased size, and it was confirmed that the overall uniformity of Cu doping increases based on the cross section of the secondary particles.

[0089] As a result, Cu was doped non-uniformly. However, referring to subsequent experimental examples, it was confirmed that the O3-type crystal structure was well maintained and that the outflow (release) of Na from within the lattice structure was prevented after washing the positive electrode active material with water.

[0090] Experimental Example 2: Comparison of positive electrode active material particles before and after washing with water The positive electrode active material particles prepared in Example 1 and Comparative Examples 1 to 3 were subjected to comparative SEM and XRD analysis before and after washing with water, and the results are shown in FIGS. 2a, 2b and 3. FIG.

[0091] 2a and 2b, when observing the surface shape by SEM, it was confirmed that some of the secondary particles were cracked after washing with water, and it was confirmed that the particle cracking was the least in Example 1, and most of the secondary particles were cracked in Comparative Example 3.

[0092] 2b, it can be seen that in Example 1, by carrying out the oxidizing roasting process, the average particle size (D1) (D50) of the multiple primary particles constituting the secondary particles is in the range of 800 nm to 2.5 μm, the average particle size (D2) (D50) of the secondary particles is in the range of 6 to 12 μm, the aspect ratio of the primary particles is in the range of 1:1 to 1:2.5, and the average particle size ratio (D2 / D1) is in the range of 2.5 to 10. On the other hand, in Comparative Examples 1 to 3, the average particle size of the primary particles is smaller than that of Example 1, and the aspect ratio of the primary particles is higher, so that the primary particles are needle-like or rod-like, and the average particle size ratio is relatively high at more than 10.

[0093] [Table 1]

[0094] 3 and Table 1, the XRD titration results showed that the (003) peak, which is the main peak of O3-type, appeared in the 2θ range of 15° to 17.5°, and it was confirmed that the O3-type crystal structure was maintained after washing with water in Example 1. Specifically, the full width at half maximum of the (003) peak (FWHM(003)) was measured to be 0.1847.

[0095] On the other hand, in Comparative Examples 1 to 3, the FWHM(003) increased to 0.27 to 0.66, the O3-type crystal structure collapsed after washing with water, and most of the particles were cracked in Comparative Example 3, which had the highest FWHM(003).

[0096] Moreover, in Comparative Example 4 in which Co was doped instead of Cu, it was confirmed that most of the positive electrode active material particles were cracked after washing with water.

[0097] Experimental Example 3: Measurement of Residual Na Content (TTS) In Example 1 and Comparative Examples 1 to 3, the residual Na content on the surface of the positive electrode active material after washing with water was measured, and the results are shown in FIG.

[0098] The residual sodium content was determined by potentiometric titration of the residual Na-containing compound (e.g., NaOH or Na 2 CO 3 ) and then the total amount of Na alone was calculated separately to obtain the value (TTS, Total Sodium).

[0099] The calculation method is as shown in the following formula 1. Formula 1

[0100] TTS(Total Na)=NaOH analysis value(%)×Na / NaOH+Na 2 CO 3 Analysis value (%) x 2Na / Na 2 CO 3

[0101] 4, it was confirmed that in Example 1 and Comparative Examples 1 to 3, the residual Na content (TTS) after washing with water was low, at around 3,000 ppm.

[0102] Experimental Example 4: Evaluation of battery performance The sodium secondary batteries manufactured in Example 1 and Comparative Examples 1 to 3 were subjected to charge-discharge experiments at 25° C., voltage range of 2.0 V to 4.6 V, and discharge rate of 0.1 C to 2.0 C using an electrochemical analyzer (Toyo, Toscat-3100) to measure initial charge capacity, initial discharge capacity, initial reversible efficiency, and rate capability (C-rate).

[0103] In addition, the same sodium secondary battery was charged / discharged 50 times at 25°C and 1C / 1C within a driving voltage range of 2.5V to 4.3V, and then the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention) was measured.

[0104] The measurement results are shown in Table 2 below.

[0105] [Table 2]

[0106] Referring to Table 2, it was confirmed that Example 1 had the best electrochemical properties.

[0107] As described above, the present invention has been illustrated and described with reference to specific embodiments. However, it will be apparent to those skilled in the art that the present invention can be modified and changed in various ways without departing from the technical spirit of the present invention as defined by the following claims.

Claims

1. The present invention includes an O3 type sodium complex transition metal oxide including at least sodium, a transition metal, and a doping metal, The sodium composite transition metal oxide is a secondary particle formed by agglomeration of a plurality of primary particles, and the aspect ratio of the primary particles is 1:1 to 1:2.

5.

2. 2. The positive electrode active material for sodium secondary batteries according to claim 1, wherein the ratio (D2 / D1) of the average particle diameter (D2) of the secondary particles to the average particle diameter (D1) of the primary particles is 2.5 to 10.

3. 2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the primary particles have an average particle size D1 of 0.8 to 2.5 μm, and the secondary particles have an average particle size D2 of 6 to 12 μm.

4. The positive electrode active material for a sodium secondary battery according to claim 1 , wherein the doping metal is copper (Cu).

5. The positive electrode active material for a sodium secondary battery according to claim 1 , wherein the sodium composite transition metal oxide is represented by the following Chemical Formula 1: 【Chemistry 1】 In the above Chemical Formula 1, TM is at least one selected from Co, Ni, Mn, and Fe; M is at least one selected from P, Sr, Ba, Ti, Zr, W, Co, Mg, Al, Zn, Ce, Hf, Ta, F, Cr, V, Si, Y, Ga, Sn, Mo, Ge, Nd, B, Nb, and Gd; 0.8<a<1.0, 0.01≦x≦0.1, 0≦y≦0.1, and 0.8≦1−x−y≦0.

99.

6. The positive electrode active material according to claim 1, characterized in that the (003) peak half width (FWHM(003)) at 2θ of 15° to 17.5° in XRD analysis is 0.1599 to 0.3399.

7. 2. The positive electrode active material for a sodium secondary battery according to claim 1, wherein the positive electrode active material has a residual Na content (TTS, Total Sodium) of 100 to 3,000 ppm.

8. roasting the transition metal hydroxide precursor; dry mixing the roasted precursor prepared in the roasting process with a doping metal compound; mixing the metal-doped roasted precursor prepared in the dry mixing process with a sodium compound in an equivalent amount (Na / M (total metals excluding Na)) of more than 0.8 and less than 1), and then calcining the mixture.

9. The method for producing a positive electrode active material for a sodium secondary battery according to claim 8, wherein the roasting process is performed at a temperature of 700 to 1,100° C. in an oxidizing atmosphere.

10. In the dry mixing step, The method for producing a positive electrode active material for a sodium secondary battery according to claim 8, wherein the doping metal compound is a copper (Cu) acetate compound, a sulfide, a nitride, a phosphide, an oxide, an oxyhydroxide, a hydroxide, or a combination thereof.

11. The method for producing a positive electrode active material for a sodium secondary battery according to claim 8, wherein the calcination step is carried out at a temperature of 800 to 1,100° C.

12. The method for producing a positive electrode active material for a sodium secondary battery according to claim 8, further comprising a step of washing the sodium transition metal oxide produced in the calcination step with water.

13. A positive electrode for a sodium secondary battery comprising the positive electrode active material according to claim 1.

14. A sodium secondary battery comprising the positive electrode and the negative electrode according to claim 13.

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